The present invention relates generally to the field of hydraulic fracturing.
"Hydraulic fracturing" is the name of a commonly used oilfield operation, and it consists of the forcing of fluid-particle mixtures into the well pipe at high pressures and rates. This fluid-particle mixture travels down the pipe and into the oil or gas producing earth formation through appropriate openings in the form of perforations, or the like, in the well pipe. Technical data obtained from a large number of hydraulic fracturing operations have proven that earth formations tend to part or split, when fluids at high pressure and rates are pumped into them. The mixture of liquid and particles travels into the split or "fracture" as it is called. When pumping is halted and some of the fluid from the fluid-particle mixture "bleeds-off" or migrates to the formation, the formation split closes until it touches the particles. The particles remaining in the fracture, if properly designed, can prevent the split or fracture from completely closing. These remaining particles are herein referred to as "the proppant layer". Its function is to provide a layer of particles through which formation fluids will more easily flow to the wellbore, and thence flow or be artificially lifted or pumped to the surface. The hydraulic fracturing process has been used successfully heretofore to stimulate oil and gas production from thousands of wells, since the beginning of its general usage in the 1950's. The process has proved to be successful in many formations; however, only those formations which have certain characteristics have been profitably fractured. In general, economic stimulation by fracturing has occurred mainly in the harder formations, and softer formations have rarely been effectively stimulated. Large numbers of hydraulic fracturing treatments have been performed on wells completed from softer formations, and a high percentage of failures has occurred in spite of the successful displacement of the fluid-particles mix into the formation. In hard formations, effective stimulation has been achieved both with proppant layers which are "packed" and those which are "monolayered". A packed proppant layer is one in which the proppant layer, when in place, consists of several thicknesses of particles, while a monolayered proppant is one in which the proppant layer is only a single particle in thickness. Monolayers are effective in hard formations for the reason that the sides of the formation split are hard, and relatively few particles are capable of holding the split open. In contrast, a soft formation split cannot be held open by a monolayer because the proppant particles penetrate into the soft unconsolidated walls of the formation, when the split is closed by overburden effects. This action can completely close a formation split. For this reason, efforts to stimulate unconsolidated formations in general have been oriented to packed proppant layers. Packed proppant layers are relatively thin and may range from perhaps a centimeter or two in thickness near the wellbore to a sand grain's thickness at their end. Over most of their length, they are at least several particles thick. When formation particles are stopped by outer particles of the proppant layer, the openings between the inner particles are available to flow fluids.
In spite of the theoretical feasibility of stimulation of softer formations by the placement of packed proppant layers, a high percentage of wells, which have been hydraulically fractured to achieve a packed proppant layer, have failed to stimulate production. Many of the wells whose production was stimulated were successful because of the removal of wellbore damage rather than effective functioning of the packed proppant layer. "Wellbore damage" is a term used to collectively account for reduction of formation flow capacity in the immediate vicinity (typically within a meter or less) of the well pipe perforations. Such damage can be caused by filling of the formation openings by particles or viscous fluids originating either in the formation, or from outside the formation such as the fluid used to drill the well. Sand laden, high velocity mixes of particles and fluid of hydraulic fracturing treatments are effective in removing formation damage. Stimulation of production from a treated well can occur even though the proppant layer is ineffective and incapable of transmitting formation fluids to the wellbore. It should be understood that removal of wellbore damage, while desirable, is no substitute for placement of an effective proppant layer. Removal of all wellbore damage can improve well productivity to the maximum natural productivity of the formation; while in contrast, an effective proppant layer will improve well productivity to several times the maximum natural productivity of the formation. Failure of proppant layers to provide permeable paths to the wellbores in softer formations has been technically perplexing and has limited economic recovery from lower productivity softer formations.
Formations which contain valuable fluids in their pore spaces vary widely in their character. They range from very hard formations, such as limestones and sandstones whose particles are cemented, i.e., firmly held together by natural binders, to softer sandstones and unconsolidated sands whose grains are weakly cemented or completely uncemented.
Many factors in the formation (mechanisms) are of significance with respect to hydraulic fracturing. Two significant ones are overburden pressure and temperature. High pressure and temperature acting at deeper depths cause formations to perform differently than at shallow depths where temperature and pressure effects are low. Formation temperature and confining formation pressure, in addition to gravity forces, contribute materially to the ability of the formation particles to move, particularly when fluids are moving through the formation. Movement of an individual particle in the formation changes the contact points and forces holding the surrounding particles in place. These particles are free to shift and move until they reach a new, more stable arrangement. In this manner a shifting of particles occurs which makes unconsolidated formations behave in a plastic manner. At deeper depths, higher temperature and confining pressure increase the ability of formation particles to move. For this reason, the term "softer formations" as used in the description of this invention, is a relative one. A formation at low overburden pressure and temperature might be successfully stimulated by a hydraulic fracturing treatment designed for hard formations. The same formation buried deeply, where overburden pressure and formation temperature are sufficiently high, could not be stimulated by the same treatment. When a formation behaves plastically, it is considered a softer formation from a hydraulic fracturing point of view, because its particles can gradually move.
Softer formations capable of producing gas, oil, or other liquids are usually composed of sand sized particles and lesser amounts of other particles; silt sized particles of various minerals and clay particles being most common. The presence of these small particles reduces permeability because they occupy the openings which would otherwise be available for fluid flow. When the small particles are present in substantial percentages, permeability is significantly reduced. Also, in some formations the smaller particles are very mobile and readily migrate through the openings between the particles comprising the formation. When these small particles move to the vicinity of the wellbore, they may collect to cause an area of restricted permeability around the well. In some otherwise permeable formations, this severe productivity restriction, called wellbore damage, can occur a few hours after production starts; while in others, it may occur gradually over several months or several years. Formations with restricted permeability or those damaged by mobile small particles are in need of the increased productivity which can be provided by successful hydraulic fracturing. Yet it is these formations which contain large numbers of the small particles capable of invading a proppant layer and destroying its permeability.
Applicant has determined from extensive laboratory tests of unconsolidated sands containing small particles, that the small particles do move into a proppant layer containing particles such as heretofore used in hydraulic fracturing. Small formation particles will therefore tend to move through openings, when subjected to the force of moving well fluid, unless they wedge or bridge on the openings. "Wedging" is herein defined as the condition which exists when a particle does not pass through an opening because it is larger than the opening. "Bridging" is the mechanism which exists when a particle does not pass through an opening larger than it is, due to interaction with one or more other particles smaller than the opening. Bridging is effected when moving particles touch and become immobilized in a configuration which withstands forces affecting the particles.
Bridging can occur between the first particles which encounter an opening; but it is normal for some particles to pass before bridging occurs. The number of particles which pass into a proppant layer before briding occurs depends on the shapes of the openings between the proppant particles and the shapes of the small formation particles. If a proppant layer is thin, passage of relatively few formation particles into its center will sharply reduce its permeability. A proppant layer, which passes large numbers of formation particles, also experiences wellbore damage when the formation particles collect in the vicinity of the wellbore. For these reasons, halting of formation particles at the proppant layer-formation boundary by wedging rather than bridging, is necessary for optimum hydraulic fracturing of softer formations.
"Screen Fracture Proppant" (abbreviated SFP) as used herein, relates to the concept and method of providing a particle mix which screens or excludes formation particles as a function of the size of the openings between its particles. Screen Fracture Proppants differ markedly from the proppants heretofore used in fracturing operations. Conventional proppant systems are constructed to maximize permeability. Large particles possess higher permeability than small particles; consequently, conventional proppants usually consist of one or two particles whose size is large in relation to the small formation particles in the formation. In contrast, SFP, according to the present invention, usually consists of several particles each larger than the next smaller particle by a specified ratio of diameter. The SFP is constructed so that the size of all its openings are equivalent to those of the smallest particles in the mix, even when these are large particles in the mix. The size of these small openings between particles of a SFP are directly related to the size of the smallest particles contained in the formation. No other proppant particle system or concept is sized in this manner.
In contrast to the SFP concept above, U.S. Pat. No. 2,905,245, dated Sept. 22, 1959, discloses a method of packing the annular space between a liner and an unconsolidated sand formation containing a fluid by positioning a widely-graded sand pack, generally referred to as a gravel packing mix, having a distribution of sand particles substantially similar to the size distribution of particles in the formation, but each selected particle of the distributed pack having a diameter several times the diameter of the particle in the original formation to which it corresponds. While this patent uses a mixture of particles, the particle size selection is not made on the same basis as in the present invention.
More specifically, the method of this patent specifies a mix of gravel pack particles of a diameter no larger than eight times the diameter of formation particles at the 50 cumulative weight percent point, and no larger than twelve times the diameter of formation particles at the 90 cumulative weight percent point. It also specifies that the different sizes of pack particles should be present in approximately the same weight percentage as the corresponding formation size. For a pack particle no more than twelve times the size of the formation particle at the 90 percent cumulative point, the pack particle would consist of approximately 10 percent of the total mix.
The quantity of smallest particles in a mix to be used for fracturing must be present in much larger amounts than the ten percent specified in the graded sand pack method of this patent. SFP specifies 26 percent or more of the smallest particles and typically uses 45 to 75 percent. The graded pack will not adequately exclude the small particles of a formation, even if the quantity of its smallest particles are more than 26 percent, because the openings between its particles are too large. The use of pack particles that are twelve times the diameter of the formation particles at the 50 percent cumulative weight point is a satisfactory gravel packing design, but is unsatisfactory for fracturing proppant designs because its openings are excessively large. No relationship at the 90 percent cumulative weight point between the pack and formation particles according to the foregoing patent is satisfactory for compounding fracturing proppants. I have determined that the smallest particles of SFP mixes are very commonly smaller than the formation particles themselves at the 90 percent cumulative weight point.
A proppant layer constructed according to SFP specifications as outlined in the present invention possesses the highest possible permeability of any mix with openings sized to exclude, by wedging, a specific size of formation particle. SFP also can possess interwedging capability between its particles, a property no other system possesses.